Schematic representation of the human IL-1RN gene. The IL-1RN gene contains six exons, which are represented by boxes. The icIL-1Ra1 and icIL-1Ra2 mRNAs are transcribed from a promoter (Pic) located upstream of exon ic1. The icIL-1Ra1 transcript contains exon ic1 spliced to an internal splice acceptor site within exon 1, followed by exons 2 to 4. The icIL-1Ra2 mRNA contains an additional exon (ic2) inserted downstream of exon ic1. This differential splicing is symbolized by dotted lines. The sIL-1Ra mRNA is transcribed from a promoter (Ps) located immediately upstream of exon 1 and contains exons 1 to 4. The locations of PCR primers A–D and G (Supplementary Table 1) are indicated by arrows. The combination of primers A and C allows specific amplification of icIL-1Ra1 and icIL-1Ra2 cDNAs, while primers B and C or B and G specifically amplify sIL-1Ra mRNA. The combination of primers B and D allows the detection of unspliced primary sIL-1Ra transcripts.